Cathode and electrolyte materials for solid oxide fuel cells and ion transport membranes

ABSTRACT

Novel cathode, electrolyte and oxygen separation materials are disclosed that operate at intermediate temperatures for use in solid oxide fuel cells and ion transport membranes based on oxides with perovskite related structures and an ordered arrangement of A site cations. The materials have significantly faster oxygen kinetics than in corresponding disordered perovskites.

RELATED APPLICATION

This application is a nationalization of PCT Application Serial No.PCT/US06/31234, filed 9 Aug. 2006, which claims priority to and thebenefit of U.S. Provisional Patent Application Ser. No. 60/706,836 filed9 Aug. 2005.

GOVERNMENTAL SPONSORSHIP

This invention was made with government support under DE-FC26-03NT41960awarded by Department of Energy. The United States government hascertain rights in the invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to novel cathode and electrolyte materials forintermediate temperature solid oxide fuel cells and ion transportmembranes based on oxides having perovskite related structures and anordered arrangement of A site cations.

More specifically, the present invention relates to a family ofcompositions including at least one compound having the general formula(I):(ABO₃)_(p)(A′BO_(2+x))_(q)(A′O_(2+x))_(r)  (I)where p, q, and r are integers, x is a real number, A is a divalentmetal, A′ is a trivalent metal and B is a metal capable of readyconversion between its plus three oxidation state and its plus fouroxidation state or mixtures or combinations of such metals and where thecathode compositions possess both oxygen ion diffusivity and electronicconductivity. Cathode compositions also find applications in iontransport oxygen separation membranes and cathode.

More specifically, the present invention also relates to a family ofelectrolyte compositions including at least one compound having thegeneral formula:(AB′O₃)_(p)(A′B′O_(2+x))_(q)(A′O_(2+x))_(r)  (II)where p, q, and r are integers, x is a real number, A is a divalentmetal, A′ is a trivalent metal and and B′ is a non transition metal andwhere the electrolyte compositions is a pure ionic conductor.Electrolyte compositions can be used in chemical sensors.

More specifically, the present invention also relates to fuel cells andseparation cells including a composition of formulas (I) and/or (II) andthe methods for making and using same.

2. Description of the Related Art

Solid oxide fuel cells (SOFCs) have the promise to improve energyefficiency and to provide society with a clean energy producingtechnology. The high temperature of operation enables the solid oxidefuel cell to operate well with existing fossil fuels and to be used incombined heat and power applications or efficiently coupled withturbines, to give very high efficiency conversion of fuels toelectricity. SOFCs are quiet and non-polluting and their inherent highefficiency leads to lower greenhouse gas emissions.

At present, solid oxide fuel cells operating at 1000° C. utilize ayttria stabilized zirconia electrolyte (YSZ), a lanthanum strontiummanganite cathode, and a nickel-YSZ cermet anode. Cells are connected bya lanthanum strontium chromite bipolar plate. Various geometries for thecell design have been investigated, but the most developed is theSiemens-Westinghouse tubular configuration in which the YSZ electrolytefilm (30-40 μm) is supported on a 1.5 m long tube of porous lanthanumstrontium manganite. The Siemens-Westinghouse design has beendemonstrated successfully at the 100 kW scale.

While the technology has been successfully established, costs remain toohigh to permit widespread introduction of SOFCs into the marketplace.Cost reduction requires both improvements in the properties of thematerials, particularly the electrodes, and the development ofinexpensive fabrication processes. Lowering the operating temperaturehas a significant impact on cost by allowing the use of less expensivematerials in interconnects and heat exchangers. Lower temperatures alsolead to an increase in the reliability of SOFC systems by reducingproblems associated with thermal cycling and performance degradation dueto inter-diffusion or reaction of the individual components.Intermediate temperature SOFCs in both planar and tubular configurationsat the 3-10 kW scale, for distributed combined heat and power and forauxiliary power are currently being developed in the U.S. by severalorganizations.

Operation of SOFCs at intermediate temperatures (500-800° C.) requiresnew combinations of electrolyte and electrode materials that willprovide both rapid ion transport across the electrolyte andelectrode-electrolyte interfaces and efficient electrocatalysis of theoxygen reduction and fuel oxidation reactions.

Mixed ionic electronic conducting (MIEC) oxides are the major functionalcomponent of ion transport membranes (ITMs). ITMs operate at hightemperature by catalyzing the dissociation and reduction of an oxygenmolecule at one membrane surface followed by coupled transport of anoxygen ion and two electron holes through the bulk material. On thesecond surface, the oxide ion acts as an oxidant for the catalyzedreaction of hydrocarbons such as methane to form synthesis gas orrecombines to give molecular oxygen releasing electrons back into themembrane. In the latter case, the membrane functions as a hightemperature oxygen separation device. Ion transport membrane systems aresimpler in design than SOFCs in that no external circuit is required,but at the same time the materials requirements are very demandingbecause of the large gradient in oxygen activity across the membrane.The overall performance of an ITM is determined by the bulk transportand surface reactions which are strongly coupled together.

Thus, there is a need in the art for a new class of materials that willallow SOFCs and ITMs to operate at intermediate temperatures.

SUMMARY OF THE INVENTION

This invention provides a new class of cathode compositions havingsuperior properties for oxygen reduction that are suitable for use atintermediate temperatures in a range between about 400° C. and about800° C.

This invention provides a new class of electrolyte compositions havingsuperior properties at intermediate temperatures in a range betweenabout 400° C. and about 800° C.

The invention also provides a new class of membrane forming compositionhaving superior properties for use in oxygen separation.

These compositions solve the problem of reducing operating costs becausethe compounds in the compositions allow operation of SOFCs and ITMs atintermediate operating temperatures in a range between about 400° C. andabout 800° C.

This invention provides a class of perovskite oxides cathodecompositions including at least one compound having the general formula(I):(ABO₃)_(p)(A′BO_(2+x))_(q)(A′O_(2+x))_(r)  (I)where p, q, and r are integers, x is a real number, A is a divalentmetal, A′ is a trivalent metal cation and B is a transition metal cationor mixtures or combinations of transition metals and where the cathodecompositions have unusually large oxygen ion diffusion coefficientsmaking them ideally suitable as electrodes for intermediate temperaturesolid oxide fuel cells and ion transport membranes for oxygenseparation. The value of p generally ranges from 1 to about 4. When thevalue of p is 4, then the value of q ranges from 1 to 4 and the value ofr ranges from 0 to 4, i.e., p≧q (p is greater than or equal to q) andq≧r (q is greater than or equal to r) requiring that p≧r (p is greaterthan or equal to r). In certain embodiments, the compositions aredefined as having p>q (p is greater than q), q>r (q is greater than r),which requires that p>r (p is greater than r). In other embodiments, pis greater than 4, which of course enlarges the corresponding ranges ofq and r. The value of x is a real number having a range greater than 0.0and less than 1.0, i.e., 0.0<x<1.0.

This invention provides a class of perovskite oxides electrolytecompositions including at least one compound having the general formula(II):(AB′O₃)_(p)(A′B′O_(2+x))_(q)(A′O_(2+x))_(r)  (II)where p, q, and r are integers, x is a real number, A is a divalentmetal, A′ is a trivalent metal cation and B′ is a non transition metalor mixtures or combinations thereof making them ideally suitable forelectrolytes that are capable of efficient operation in intermediatetemperature solid oxide fuel cells. The value of p generally ranges from1 to about 4. When the value of p is 4, then the value of q ranges from1 to 4 and the value of r ranges from 0 to 4, i.e., p≧q (p is greaterthan or equal to q) and q≧r (q is greater than or equal to r) requiringthat p≧r (p is greater than or equal to r). In certain embodiments, thecompositions are defined as having p>q (p is greater than q), q>r (q isgreater than r), which requires that p>r (p is greater than r). In otherembodiments, p is greater than 4, which of course enlarges thecorresponding ranges of q and r. For a single non-transition metalhaving a plus three oxidation state, e.g., Al³⁺, x is exactly equal to0.5. To permit x to vary in the range between x having a value greaterthan 0.0 and less than 1.0, i.e., 0.0<x<1.0, the composition includes amixture of trivalent non-transition metals and tetravalentnon-transition elements, e.g., Al³⁺ and Ti⁴⁺. The compounds of formula(II) having x in the range between 0.0>x<1.0 are generally formed byalio valent doping, a process where some trivalent metals are related bytetravalent metals.

The present invention provides a fuel cell including a hydrogenreservoir in contact with an anode comprising a known anode composition,an oxygen reservoir in contact with a cathode comprising a compositionincluding at least one compound of formula (I) and an electrolyteinterposed therebetween comprising a composition including a compound offormula (II) or a traditional or known fuel cell electrolytic material.

The present invention provides an oxygen enrichment apparatus includingan oxygen containing gas reservoir and an oxygen reservoir with anoxygen diffusion membrane comprising a composition including at leastone compound of formula (I) interposed therebetween.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention can be better understood with reference to the followingdetailed description together with the appended illustrative drawings inwhich like elements are numbered the same:

FIG. 1A depicts a front view of the structure of a compound of formula(I), where p=q=1 and r=0 and where the white spheres are oxygen latticesites, the dark gray sphere are B metal lattice sites, the gray spheresare partially filled oxygen ion lattice sites and the grey square areunfilled oxygen lattice sites (A and A′ are not shown as these atoms areeither in front or behind shown sites);

FIG. 1B depicts a perspective view of structure of FIG. 1A, where theblack represent the barium cations (A) and light gray spheres representpraseodymium cations (A′), the square pyramidal coordination representcobalt ions (B) with the small black sphere representing oxygen latticesites, and the small dark gray spheres represent partially occupiedoxygen ion sites;

FIG. 1C depicts a front view of the structure of a compound of formula(I), where p=q=r and where the red spheres are oxygen lattice sites, theblue sphere are B metal lattice sites, the gray spheres are partiallyfilled oxygen ion lattice sites and the squares unfilled oxygen latticesites (A and A′ are not shown as these atoms are either in front orbehind shown sites);

FIG. 1D depicts a front view of the structure of a compound of formula(I), where p=2, q=1, and r and where the red spheres are oxygen latticesites, the blue sphere are B metal lattice sites, the gray spheres arepartially filled oxygen ion lattice sites and the squares unfilledoxygen lattice sites (A and A′ are not shown as these atoms are eitherin front or behind shown sites);

FIG. 2 depicts the DC conductivity of PrBaCo₂O_(5.5+x) andNdBaCo₂O_(5.5+x);

FIG. 3 depicts the oxygen diffusion and surface exchange coefficientsfor PrBaCo₂O_(5.5+x) and NdBaCo₂O_(5.5+x);

FIG. 4 depicts an X-Ray diffraction pattern with Rietveld refinement ofPrBaCo₂O_(5+x) with cell parameters, a=3.9084(1) Å, b=3.9053(1) Å,c=7.6343(2) Å;

FIG. 5 depicts thermogravimetric data for PrBaCo₂O_(5+x) showing oxygenstoichiometry as a function of temperature at the oxygen partialpressures indicated;

FIG. 6 depicts Variation of the oxygen partial pressure with the oxygenatom concentration (c_(O)) in PBCO at various temperatures.

FIG. 7 depicts Total conductivity of PrBaCo₂O_(5.5+x) at 0.01 (▪) and0.21 (●) atm, open and closed symbols correspond to measurements onheating and cooling.

FIG. 8 depicts The temperature dependences of D_(chem) and k_(chem)measured by ECR for PrBaCo₂O_(5+x).

FIGS. 9A-C depicts IEDP results for PrBaCo₂O₅, infused at 400° C. for 5min in 0.2 atm ¹⁸O₂ (99%) a: secondary electron image withPrBaCo₂O_(5.5+x) region on the right and epoxy matrix on left. b SIMSimage of the fraction of ¹⁸O, f¹⁸O=(¹⁸O/(¹⁸O+¹⁶O)) of the same region asin panel a. c: the profile of f¹⁸O derived from the image in panel b

FIG. 10 depicts Comparison of the values of D_(O) measured by ECR forceramic samples

FIG. 11 depicts Comparison of the values of k_(O) measured by ECR forceramic samples

FIG. 12 depicts Area specific resistance a function of temperature for asymmetric CGO-PBCO/CGO/CGO-PBCO cell

FIG. 13 depicts a hydrogen/oxygen fuel cell of this invention;

FIG. 14 depicts V-I characteristics of the CGO electrolyte supportedsolid oxide fuel cell with a PBCO cathode;

FIG. 15 depicts open circuit voltages of a CGO/PBCO electrolytesupported solid oxide fuel cell;

FIG. 16 depicts power densities of the cell as a function of currentdensity;

FIG. 17 depicts a comparison of the ohmic resistance of the cellmeasured by current interrupt technique and the calculated theoreticalresistance of the electrolyte; and

FIG. 18 depicts a block diagram of an oxygen separation apparatus ofthis invention.

LIST OF ACRONYMS AND ABBREVIATIONS OF THE INVENTION

The term CGO means Cerium Gadolinium Oxide.

The term ECR means Electrical Conductivity Relaxation.

The term EPMA means Electron Probe MicroAnalysis.

The term GDC means Gadolinia Doped Ceria (see CGO).

The term LSGM means Lanthanum Strontium Magnesium Gallate.

The term PLD means Pulsed Laser Deposition.

The term TGA means Thermogravimetric Analysis.

The term YSZ means Yttria Stabilized Zirconia.

The term IEDP means Isotope Exchange and Depth Profiling.

The term PBCO means Praesodymium Barium Cobalt Oxide.

The term STO means Strontium Titanate.

The term LAO means Lanthanum Aluminate.

DETAILED DESCRIPTION OF THE INVENTION

The inventors have found that a new class of perovskite oxides can beconstructed that have enhanced oxygen diffusivity and conductivityproperties ideally suited for use in intermediate temperature fuel cellsand intermediate temperature oxygen membrane separation applications andother similar applications. The inventors have also found that a newclass of perovskite oxides can be constructed for use as an electrolytefor use in intermediate temperature fuel cell applications by changingthe readily convertable trivalent metal ions (readily convertiblebetween their plus three oxidation state and their plus four oxidationstate, i.e., M³⁺

M⁴⁺) in the lattice of the perovskite oxides described above to metalions that are not readily convertible between their plus three and plusfour oxidation states.

The inventors have found that particular perovskite oxides with orderedA cations which, in turn, localize the oxygen vacancies into layers canbe constructed with improved conductivity and oxygen diffusivity in anintermediated temperatures range between 400° C. and 800° C. One memberof this class of compounds has the general formula:AA′B₂O_(5+x)where A′=Ba, and A=Y, or a trivalent lanthanide ion, and B is a firstrow transition metal ion, excluding Sc, Ti and Zn. The ideal structureof this family of compounds is generated by a stacking sequence: . . .|A′O|BO₂|AO_(x)|BO₂| . . . , to form structures closely related to thecuprate superconductors. The vacant sites are arranged so that when x=0,the A cation is eight coordinate. The structure of PrBaCo₂O₅, (PBCO), amember of this new class of perovskite oxides is shown in FIGS. 1A&B.

The ordered vacancies in these materials are thought to cause a greatenhancement in the diffusivity of oxide ions in the bulk of the materialand may supply surface defect sites with enhanced reactivity towardsmolecular oxygen. The low temperature structural properties of thesecompounds have been studied in detail (W. Zhou, C. T. Lin, W. Y. Liang,Adv. Mater. 5 1993 735; I. O. Troyanchuk, N. V. Kasper, D. D. Khalyavin,H. Szymczak, R. Szymczak, M. Baran, Phys. Rev. Lett. 80 1998 3380; A.Maignan, C. Martin, D. Pelloquin, N. Nguyen, B. Raveau, J. Solid StateChem. 142 1999 247; P. S. Anderson, C. A. Kirk, J. Knudsen, I. M.Reaney, A. R. West, Solid State Sciences 7, 2005, 1149; and S. Streule,A. Podlesnyak, J. Mesot, M. Medarde, K. Conder, E. Pomjakushina, E.Mitberg and V. Kozhevnikov J. Phys.: Condens. Matter 17, 2005, 3317),but little is known concerning their high temperature oxygen chemistry.

The inventors have also found that measurements of the surface oxygenexchange kinetics and bulk oxide ion diffusion coefficients forLnBaCo₂O_(5+x) (Ln=Pr, Nd) by isotope exchange and depth profiling(IEDP) and electrical conductivity relaxation (ECR) indicate that thesematerials are ideally suited for use as intermediate temperature cathodematerials for fuel cell application, as intermediate temperatureelectrolyte materials and as intermediate temperature oxygen separationmaterials. Both techniques demonstrate that the oxygen kinetics in thesestructure type are significantly faster than in corresponding disorderedperovskites.

The compositions of this invention for use as cathode materials andoxygen diffusion materials broadly comprise at least one compound havingthe general formula (I):(ABO₃)_(p)(A′BO_(2+x))_(q)(A′O_(2+x))_(r)  (I)where the compounds has a perovskite oxide structure, p, q, and r areintegers, x is a real number, A is a divalent metal cation, A′ is atrivalent metal cation and B is a transition metal cation or mixtures orcombinations thereof. The A′ trivalent metal cations are convertibleinto their tetravalent state accounts for x being a real number. Thevalue of p generally ranges from 1 to about 4, but higher values arepossible. When the value of p is 4, then the value of q ranges from 1 to4 and the value of r ranges from 0 to 4, i.e., p≧q and q≧r requiringthat p≧r. In certain embodiments of the compositions, p is greater thanq, a q is in turn greater than r requiring p to be greater than r (p>q,q>r and p>r). In certain other embodiments, p is greater than 4, whichin turn enlarges the corresponding ranges of q and r. The value of xranges form a real number greater than 0.0 and less than 1.0, i.e.,0.0<x<1.0, which is controlled by the amount of A′ metals that areconverted from their plus three state to their plus four state or visaversa. The compositions are capable of lowering the operatingtemperature of fuel cells from the current operating temperatures whichare above 800° C. to an intermediate temperature between about 400° C.and about 800° C. In certain embodiments, the operating temperature isbetween about 400° C. and about 700° C. In other embodiments, theoperating temperature is between about 400° C. and about 600° C. Inother embodiments, the operating temperature is between about 450° C.and about 800° C. In other embodiments, the operating temperature isbetween about 450° C. and about 700° C. In other embodiments, theoperating temperature is between about 450° C. and about 600° C.

The compositions of this invention for use as cathode materials andoxygen diffusion materials broadly comprise at least one compound havingthe general formula (I):(AB′O₃)_(p)(A′B′O_(2+x))_(q)(A′O_(2+x))_(r)  (I)where the compounds has a perovskite oxide structure, p, q, and r areintegers, x is a real number, A is a divalent metal, A′ is a trivalentmetal cation and B′ is a non transition metal or mixtures orcombinations thereof for electrolyte compositions. The value of ppreferably ranges from 1 to about 4. When the value of p is 4, then thevalue of q ranges from 1 to 4 and the value of r ranges from 0 to 4,i.e., p≧q and q≧r requiring that p≧r. In preferred compositions, p isgreater than q which is in turn greater than r requiring p to be greaterthan r. Although p preferably ranges from 1 to 4, p can be greater than4, which of course enlarges the corresponding ranges of q and r. Thevalue of x ranges form a real number greater than 0.0 and less than 1.0,i.e., 0.0<x<1.0. The compositions are capable of lowering the operatingtemperature of fuel cells from the current operating temperatures whichare above 800° C. to an intermediate temperature between about 400° C.and about 800° C. In certain embodiments, the operating temperature isbetween about 400° C. and about 700° C. In other embodiments, theoperating temperature is between about 400° C. and about 600° C. Inother embodiments, the operating temperature is between about 450° C.and about 800° C. In other embodiments, the operating temperature isbetween about 450° C. and about 700° C. In other embodiments, theoperating temperature is between about 450° C. and about 600° C.

The compounds are comprised of three distinct building blocks: ABO₃,A′BO₂, and A′O_(2+x) which are combined in different ways to give thevarious compounds of the invention according to the following rules: p≧qand q≧r. Examples are given in Table 1.

TABLE 1 Examples of Compositions of the Present Invention p q rComposition 1 1 0 AA′B₂O_(5+x) 1 1 1 AA′₂B₃O_(7+x) 2 1 0 A₂A′B₃O_(8+x)

The structural arrangements of the building blocks are shown in FIGS.1A-D for the three examples of compounds of Formula (I) in Table 1. As aconsequence of the structural arrangement, the A and A′ cations occupyseparate layers, that is they are ordered, but not shown, except in theperspective drawing of FIG. 1B. The oxygen vacancies are therebyconfined into layers and it is this feature that is responsible for thehigh oxygen diffusivity.

The divalent metal cation suitable for use in the compounds of thisinvention suitable for cathodes and oxygen diffusion membranes include,without limitation, barium (Ba), strontium (Sr) or lead (Pb) or mixturesor combinations thereof, while the trivalent metal is, withoutlimitation, yttrium (Y) or a trivalent lanthanide element includinglanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd),promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium(Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm),ytterbium (Yb), and lutetium (Lu) or mixtures or combinations thereof.In cathode or ion transport membrane applications, electronicconductivity is required in addition to ionic conductivity in this casethe B cation is chosen from the first transition series of elementsexcluding titanium or mixtures of combinations thereof. In certainembodiments, the B cation is manganese, iron, cobalt and nickel ormixtures or combinations thereof. In other embodiments, A=barium (Ba)and A′=yttrium (Y), lanthanum (La), praseodymium (Pr), neodymium (Nd),samarium (Sm) or mixtures or combinations thereof and B=iron (Fe),cobalt (Co), or mixtures or combinations thereof. Exemplary examples ofcompounds of formula (I) are PrBaCo₂O_(5.5+x), PrBaFe₂O_(5.5+x),NdBaCo₂O_(5.5+x), and NdBaFe₂O_(5.5+x).

Electronic conductivity is detrimental to electrolyte applications andconsequently in this situation compounds of formula (II) are utilized inthe composition with B′ being a non-transition metal or titanium. Incertain embodiments, B′ is magnesium (Mg), zinc (Zn), aluminum (Al),gallium (Ga), scandium (Sc), titanium (Ti) and germanium (Ge) ormixtures or combinations thereof.

Synthesis and Processing

The compounds of this invention can be synthesized in polycrystallineform via a citrate precursor method. The resulting powders are densifiedby first pressing into a 1-inch diameter die followed by cold isostaticpressing (CIP). The green disk is sintered in air and then cooled downslowly in nitrogen. Other variants of this synthesis procedure may beemployed to synthesize the compounds of this invention, including solidstate reaction, spray pyrolysis, freeze drying and sol-gel techniques.

The compounds of the invention can be converted to porous thin or thickfilms by spraying, painting, or screen printing inks of small particlesin a suitable binder. Alternatively, physical vapor deposition methods,including pulsed laser deposition, sputtering, chemical vapordeposition, photo-assisted chemical vapor deposition, aerosol assistedchemical vapor deposition, atomic layer deposition, may be used toproduce dense or porous films on electrolyte substrates or dense filmson porous supports for ion transport membranes.

EXPERIMENTAL SECTION OF THE INVENTION

The following examples and analytical measurements are included for thesake of the completeness of the disclosure and to illustrate the presentinvention, but in no way are these examples and analytical measurementincluded for the sake of limiting the scope or teaching of thisinvention.

Example 1

This example illustrates the synthesis of PrBaCo₂O_(5+x) andNdBaCo₂O_(5+x) via a citrate precursor method. Stoichiometric amounts ofPr₆O₁₁(Alfa 99.99%) or Nd(NO₃)₂, and barium and cobalt nitrates(Aldrich, 99.99%), were dissolved in dilute nitric acid. Ethylene glycol(EM science >98%) and citric acid (Aldrich, 99.5%) were added to thesolution. This mixtures were covered and stirred at 150° C. until thesolutions began to foam and formed dry resins. Further heating at 300°C. and then 600° C. for 24 hours decomposed the dry foams containingresidual organic components. The final mixtures were pressed intopellets and sintered in air at 1100° C. for 12 hours and the cooled toroom temperature in a nitrogen atmosphere. X-ray diffraction patterns ofthe final products confirmed the formation of single phases ofPrBaCo₂O_(5+x) and NdBaCo₂O_(5+x).

Example 2

This example shows the variation of the DC conductivity ofPrBaCo₂O_(5.5+x) and NdBaCo₂O_(5.5+x). The total conductivity wasmeasured by the 4-probe method at pO₂=0.01 and 0.21 atm in the range 25°C.≦T° C.≦812° C. The measurements were made AC at a single frequency (1kHz) using a dual-phase lock-in amplifier (Stanford Instruments ModelSR830). The total conductivity was measured on rectangular bars ofPrBaCo₂O_(5.5+x) (1.3×0.19×0.15 cm) and NdBaCo₂O_(5.5+x) (1.4×0.2×0.21cm). Near ambient temperature, the conductivities reach 600 Scm⁻¹ and1000 Scm⁻¹ for the Pr and Nd compounds, respectively (FIG. 2). As thetemperature is increased, the conductivities begin to decrease at ˜150°C. due to the loss of oxygen atoms from the lattices and reduction ofCo(IV) to Co(III). The data indicate that the onset temperature foroxygen loss is slightly lower for the Nd compound. The occurrence ofoxygen loss at such low temperature indicates that the high mobility ofoxygen ions in these phases.

Example 3

This example shows the high value of the oxygen transport kinetics in ofPrBaCo₂O_(5.5+x) and NdBaCo₂O_(5.5+x) by measurement of the diffusioncoefficient and surface exchange coefficient by electrical conductivityrelaxation in the temperature range between about 300° C. and about 450°C. The measurements were made AC at a single frequency (1 kHz) using adual-phase lock-in amplifier (Stanford Instruments Model SR830).Measurements were made with an abrupt gas switch on both oxidation andreduction and were performed twice at each gas switch and temperature.The self diffusion coefficients and surface exchange coefficients areshown in FIGS. 2 and 3. These values are remarkably high at such lowtemperature in comparison with other materials.

Example 4 Synthesis of PBCO

PrBaCo₂O_(5+x) (PBCO) was synthesized via a citrate precursor method (L.W. Tai, P. A. Lessing, J. Mater. Res. 7, 1992, 511). Stoichiometricamounts of Pr₆O₁₁ (Alfa 99.99%) and barium and cobalt nitrates (Aldrich,99.99%), were dissolved in dilute nitric acid. Ethylene glycol (EMscience, >98%) and citric acid (Aldrich, 99.5%) were added to thesolution. This mixture was covered and stirred at 150° C. until thesolution began to foam and formed a dry resin. Calcination for 24 h eachat 300° C. and then 600° C. decomposed the remaining organic componentsin the foam. The final mixture was pressed into pellets in a 1-inchdiameter die followed by cold isostatic pressing (CIP). The pellet wassintered in air at 1100° C. for 12 h and then cooled down slowly(2K/min.) in nitrogen. The bulk density, measured by the Archimedesmethod, was 90% of the theoretical value. Rectangular-shaped bars (˜2mm×2 mm×10 mm) for conductivity/ECR measurements were cut from the disksalong with 4 mm×4 mm rectangular plates suitable for IEDP.

Experimental Techniques Used to Characterization PBCO

X-ray powder diffraction measurements (Scintag XDS 2000, Cu Kα) wereperformed to confirm the structure and the powder pattern was analyzedby Rietveld refinement using the GSAS program (Larson, R. B. Von Dreele,Los Alamos Laboratory Rep. No La-UR-86-748 1987). A scan rate of0.125°/min. was used with the range 5°≦2θ≦90°. Thermogravimetric (TAInstruments 2950) determination of the equilibrium oxygen stoichiometryas a function of temperature in (dry) oxygen partial pressureatmospheres was obtained within the range of 0.01≦pO₂≦0.21 atm and 25°C.≦T≦800° C. The samples were first heated to 800° C. to remove anytraces of water or surface carbonate, cooled to 25° C. at 1° C./min. andthen reheated to 800° C. Thermodynamic factors, G_(O), required forcomparison of ECR and IEDP measurements, were derived from thethermogravimetric data according to equation (1):

$\begin{matrix}{\Gamma_{o} = {\frac{{\partial\ln}\; a_{o}}{{\partial\ln}\; c_{o}} = {\frac{1}{2}\frac{{\partial\ln}\;{pO}_{2}}{{\partial\ln}\; c_{o^{2 -}}}}}} & (1)\end{matrix}$

Transport Measurements (ECR and IEDP)

The total conductivity was measured by the 4-probe method at pO₂=0.01and 0.21 atm in the range 25° C.≦T≦812° C. The measurements were made ACat a single frequency (1 KHz) using a dual-phase lock-in amplifier(Stanford Instruments Model SR830). Data acquisition and control wereperformed with the Labview (National Instruments) program. Conductivitytransients were measured in duplicate at various temperatures and forvarious oxygen partial pressure changes, which were performed both asoxidation and reduction transients. The transient response yields thesurface (k_(chem)) and bulk (D_(chem)) contributions to the rate ofchemical (oxygen) stoichiometry change during the transient (see S.Wang, A. Verma, Y. L. Yang, A. J. Jacobson, B. Abeles, Solid StateIonics 140, 2001, 125).

The transport rates of oxygen were also directly measured by ¹⁸O IEDP(see. e.g., J. A. Kilner, B. C. H Steele, L. Ilkov, Solid State Ionics12, 1984, 89 and C. A. Mims, N. I. Joos, P. A. W. van der Heide, A. J.Jacobson, C. L. Chen, C. W. Chu, B. I. Kim, S. S. Perry, Electrochemicaland Solid State Letters, 3, 2000, 59). Rectangular samples were placedon a supporting quartz rod and inserted into a quartz reactor tube heldin a tube furnace and purged by flowing oxygen (0.2 atm). Thetemperature was slowly (<5° C./min.) raised to the experimentaltemperature and held there for at least one hour, after which the gasatmosphere was rapidly switched to ¹⁸O₂ (0.2 atm, 99% isotopicabundance) and then rapidly quenched by withdrawing the support rod andsample from the reactor. Imaging time-of-flight SIMS (ToFSIMS) analysesof both the exposed surfaces and polished cross-sections of the infusedmaterials were used to determine the resulting ¹⁸O profile. Analysischamber pressures less than 1×10⁻⁹ mbar were enforced during the SIMSanalysis to avoid oxygen exchange with background gases. Values for thesurface oxygen exchange coefficient, k_(O), and the bulk oxygenself-diffusion coefficient, D_(O), were obtained by fitting simulatedprofiles to the experimental data. The relationship between thetransport parameters obtained by the IEDP and ECR is given by (k orD)_(chem) (ECR)/G_(O)=(k or D)_(O) where G_(O) is the thermodynamicfactor defined above.

Electrode Measurements

Area specific resistances of PBCO, ceria-gadolinia (CGO) compositeelectrodes on CGO were determined by AC impedance measurements usingsymmetric cells. The measurements were made in air as a function oftemperature using a Solatron 1260 impedance analyzer. Contacts were madewith gold grids and wire.

Results of the Characterization of PBCO

X-Ray Diffraction

The compounds were shown to be single phase by X-ray powder diffraction.Each powder pattern was indexed to an orthorhombic unit cell (Spacegroup=Pmmm) and refined using the Rietveld method with GSAS program.FIG. 4 shows the diffraction data along with the resulting fit anddifference pattern. The unit cell parameters a=3.9084(1)Å, b=3.9053(1)Å,c=7.6343(2)Å determined for PBCO are in agreement with previous studiesof materials cooled in air (see, e.g., P. S. Anderson, C. A. Kirk, J.Knudsen, I. M. Reaney, A. R. West, Solid State Sciences 7, 2005, 1149;S. Streule, A. Podlesnyak, J. Mesot, M. Medarde, K. Conder, E.Pomjakushina, E. Mitberg and V. Kozhevnikov J. Phys.: Condens. Matter17, 2005, 3317; and C. Frontera, A. Caneiro, A. E. Carrillo, J.Oró-Solé, J. L. Garcia-Muñoz, Chem. Mater. 17, 2005, 5439) and indicatea value of x near 0.7. The near-tetragonal symmetry (a/b=1.0008) is alsoconsistent with observations near this stoichiometry (see, e.g., S.Streule, A. Podlesnyak, J. Mesot, M. Medarde, K. Conder, E.Pomjakushina, E. Mitberg and V. Kozhevnikov J. Phys.: Condens. Matter17, 2005, 3317 and C. Frontera, A. Caneiro, A. E. Carrillo, J. Oró-Solé,J. L. Garcia-Muñoz, Chem. Mater. 17, 2005, 5439).

Thermogravimetric Analysis

The thermogravimetric data in FIG. 5 shows the average oxygenstoichiometry as a function of temperature in various oxygenatmospheres. Data obtained on heating and cooling are in good agreementand the changes in stoichiometry are reversible at temperatures above300° C.

The dependence of the oxygen partial pressure on the oxygenconcentration in the solid at the various temperatures used in the ECRmeasurements is shown in FIG. 6. The lines shown in this panel providethe best average value of the thermodynamic factor, G_(O). These valuesare summarized in Table 2. As temperature is increased, the values ofthe thermodynamic factor become smaller. Some curvature in these plotsis suggested in the FIG. 6, indicating a variation in G_(O) with oxygenpressure.

TABLE 2 Summary of the Thermodynamic Factors for PrBaCo₂O_(5.5+x) DCConductivity Temperature (° C.) G_(O) 288 193(26) 342 179(22) 397162(19) 452 150(17) 650 116(10) 700 107(9) 

The total conductivity was measured on a rectangular bar ofPrBaCo₂O_(5+x) (1.3×0.19×0.15 cm). Near ambient temperature, theconductivities reaches 2000 Scm⁻¹ as shown in FIG. 7 and as thetemperature is increased, the conductivity begins to decrease at ˜150°C. due to the loss of oxygen atoms from the lattice and reduction ofCo(IV) to Co(III).

Transport Parameters (ECR and IEDP)

The diffusion coefficients and surface exchange coefficients measuredfor PrBaCo₂O_(5+x) are shown in FIG. 8. The upper panel shows the valuesof D_(chem) and k_(chem) derived directly from the ECR measurements.Fits to the Arrhenius equation provide activation energies of 0.48 eVand 0.67 eV for D_(chem) and k_(chem), respectively. The values of theoxygen transport parameters D_(O) and k_(O) derived from the ECR datausing the values of G_(O) are shown in FIGS. 10 and 11. Also shown inFIGS. 10 and 11 are the values of D and k obtained directly from theIEDP measurements.

FIG. 9 shows ToFSIMS data from one polished cross-section infused in0.20 atmosphere O₂ (99% ¹⁸O abundance) at 400° C. for 300 seconds. Thesecondary electron and ¹⁸O fraction (f¹⁸O=¹⁸O/(¹⁸O+¹⁶O)) SIMS images(panels a and b, respectively) are shown along with the resulting ¹⁸Odepth profile from this region and its fit (panel c).

Uniform transport properties were indicated by reproducible profilesaround the full perimeters of the polished specimens, except in regionswhere gas access to the surface was blocked by the sample supportsduring infusion. A few cracks were evident in this material (not shown),but these were apparently created during quenching of the sample becauseno perturbations of the ¹⁸O fraction were seen at these crack surfaces.

FIGS. 10 and 11 compare, respectively, the D_(O) and k_(O) valuesderived from IEDP with those obtained from the ECR measurements and withtransport parameters previously reported on related materials. TheseD_(O) values from the two techniques are in good agreement. By contrast,the k_(O) values from IEDP and ECR differ substantially. The surfaceactivation rates can depend critically on subtle variations in samplepreparation and pretreatment which, in turn, affect such importantparameters as surface roughness and the surface density of defect sitesand sample to sample variability in k_(O) is a common feature of similarstudies on both ceramic and thin-film materials. The central role ofsurface defects in the oxygen activation rate has been clearly shown onhigh quality thin films of cobaltite perovskite materials. The role ofsurface defects is further illustrated by the much higher values ofk_(O) measured on these ceramic samples than those measured onhigh-quality PrBaCo₂O_(5.5+x) thin films.

Despite these uncertainties, it is clear that these double perovskitematerials exhibit unusually high activity for oxygen activation andmobility. The solid lines in FIG. 10 represent the average values ofseveral measurements by different groups on La₂NiO₄ (see J. A. Kilnerand C. K. M. Shaw, Solid State Ionics 154-155, 2002, 523; F. Mauvy, J.M. Bassat, E. Bohem, P. Dordor, J. P. Loup, Solid State Ionics 158,2003, 395; E. Boehm, J.-M. Bassat, M. C. Steil, P. Dordor, F. Mauvy,J.-C. Grenier, Solid State Sciences 5, 2003, 973; J. M. Bassat, P.Odier, A. Villesuzanne, C. Marin, M. Pouchard, Solid State Ionics 167,2004, 341; and G. Kim, S. Wang, A. J. Jacobson in preparation 2006), andECR measurements on La_(0.5)Sr_(0.5)MO_(3−x) (M=Fe, Co) (see S. Wang, A.Verma, Y. L. Yang, A. J. Jacobson, B. Abeles, Solid State Ionics 140,2001, 125 and J. Yoo, A. Verma, S. Wang, A. J. Jacobson, J. Electrochem.Soc. 152, 2005, A497).

In FIG. 11, because of the large variation in the reported values forkfor La₂NiO₄ only the ECR data are included. Comparisons with the recentmeasurements on the double perovskite, GdBaCo₂O_(5+x), are alsoinstructive. Our activation energy values are lower than those obtainedby Taskin, et al. (0.7 eV and 0.85 eV) (A. A. Taskin, A. N. Lavrov, Y.Ando, Appl. Phys. Lett. 86 2005, 091910). The pre-exponential factorsand hence the absolute values of D_(chem) and k_(chem) are much higherfor our materials than those reported in Taskin. For example, at 350°C., our values of ˜10⁻⁵ cm²s⁻¹ and ˜10⁻³ cm s⁻¹ for D_(chem) andk_(chem), respectively, are 2-3 orders of magnitude higher than thevalues reported for GdBaCo₂O_(5+x). The D_(O) and k_(O) values reportedhere, are remarkably high by comparison with other studies. For example,values of D_(O) (D*) can be obtained from the study by Taskin et al. byapplying thermodynamic factors derived from their data. G_(O) values fortheir GdBaCo₂O_(5+x) materials (based on the total oxygen content) areestimated to be in the range of 80-100 for temperatures between 400° C.and 550° C. The derived values of D for their GdBaCo₂O_(5+x) materialswould thus be at least a factor of 10 lower than our values for PBCO.Comparisons between materials are complicated by structural differences,especially crystallinity, stoichiometry and surface contamination. Forexample, in studies on thin films of simple cobaltite perovskite oxides,it was observed that less perfectly ordered materials exhibit fasterkinetics of both surface and bulk transport (X. Chen, S. Wang, Y. L.Yang, L. Smith, N. J. Wu, B.-I. Kim, S. S. Perry, A. J. Jacobson, A.Ignatiev Solid State Ionics 146/3-4, 2002 405-413). The fact that Taskinet al. studied single crystals of GdBaCo₂O_(5+x), while our materialsare polycrystalline dense ceramics is very likely a major reason for thehigher rates observed here. In studies of high quality epitaxial PBCOthin films on (100) SrTiO₃, the inventors have recently obtainedsignificantly lower values of k_(O) than observed here (see, G. Kim, S.Wang, A. J. Jacobson, W. Donner, C. L. Chen, L. Reimus, P. Brodersen, C.A. Mims. Appl Phys. Lett. 88, 2006, 024103). At 500° C., measured valuesof k_(chem) on a 200 nm film was 5×10⁻¹ cm s⁻¹—several hundred timessmaller than the values observed here on our ceramic materials. Theseexperimental data clearly evidence that these materials are ideallysuited for improved solid oxide fuel cell cathodes, oxygen permeationmembranes, and sensors.

Moreover, the rapid oxygen ion diffusion and surface exchange isreflected in the area specific resistance of electrodes containing PBCO.The data in FIG. 12 show the area specific resistance of PBCO/CGOcomposite electrodes on a CGO electrolyte measured in three separateexperiments. At 600° C., the measured value is only 0.15 ohms cm²consistent with the rapid exchange kinetics.

Fuel Cell Examples

The specific objectives of the proposed research are to develop cathodematerials that meet the electrode performance targets of 1.0 W/cm² at0.7 V in combination with YSZ at 700° C. and with GDC, LSGM or bismuthoxide based electrolytes at 600° C. The performance targets imply anarea specific resistance of about 0.5 Ωcm² for the total cell.

Further measurements on the oxygen deficient double perovskitePrBaCo₂O_(5.5+d) (PBCO) are reported. The high electronic conductivityand rapid diffusion and surface exchange kinetics of PBCO suggest itsapplication as cathode material in intermediate temperature solid oxidefuel cells (SOFCs). Examples have been completed on symmetric cells andon complete cells: Ni/CGO/CGO/PBCO/CGO.

Introduction

The present cathodic materials have the following properties: 1.0 W/cm²at 0.7 V in combination with YSZ at 700° C. and with CGO, LSGMelectrolytes at 600° C. The new cathode materials were synthesized andcharacterized and their kinetic parameters measured along with theirthermal and chemical compatibility with different electrolytes and theirsurface exchange rates, diffusion coefficients and interfacial transportvalues.

Summary

The high electronic conductivity and rapid diffusion and surfaceexchange kinetics of the oxygen deficient double perovskitePrBaCo₂O_(5.5+δ) (PBCO) make such materials ideally suitable as cathodematerial in intermediate temperature solid oxide fuel cells. Thematerial measurements on symmetric cells with PBCO electrodes had lowASR values at 600° C. as well as measurements on complete cells and forelectrolyte supported cells.

Sample Preparation

The CGO (gadolinium doped ceria) powder was die-pressed into a pelletand sintered at 1450° C. for 8 h in air. The thickness of the CGOelectrolyte pellet was reduced to 0.8 mm by polishing with SiC paper. A50:50 wt. % mixture of PrBaCo₂O_(5+x) (PBCO) and CGO (50 wt %) was usedas the cathode, and a Ni-CGO (30 vol %) material was used as the anode.The PBCO-CGO and NiO-CGO pastes were prepared from powder mixtures ofPBCO and CGO, NiO and CGO, with the addition of terpineol. The NiO-CGOpaste was applied onto the CGO electrolyte and fired at 1300° C. for 2 hin air. Then, a thin PBCO-CGO green tape (25 mm), which was prepared bytape casting, was stuck to the other side of the CGO electrolyte pelletusing terpineol and fired at 1100° C. for 30 min in air. The PBCO-CGOpaste was subsequently applied onto the surface of the sintered PBCO-CGOtape, and also fired at 1100° C. for another 2 h in air to increase thethickness of the cathode. The active electrode area of the cathode andanode was 1.27 cm². Two pieces of gold mesh were used as currentcollectors and bonded onto both surfaces of the cathode and anode byfiring with gold paste at 700° C. for 30 min in air.

Electrical Measurements

An electrolyte supported solid oxide fuel cell with the followingarrangement was tested from 500° C. to 700° C. at 25° C. intervals:

97 vol % H₂+3 vol % H₂O, Ni-CGO anode/CGO electrolyte/PBCO-CGO cathode,air

The anode side of the single cell was sealed onto an alumina tube with agold O-ring under spring loading, while the cathode was simply exposedto air. Before testing, the cell was heated to 800° C. to deform thegold O-ring and reduce the NiO to Ni with hydrogen flowing over theanode. The flow rate of the hydrogen fed to the anode was 150 cc/min.Cell current-voltage (I-V) plots were measured using an Arbin TestingSystem (Model BT 4+). The ohmic resistance of the cell was determined bycurrent interrupt measurements using Keithley SourceMeters.

Referring now to FIG. 13, an illustrative fuel cell of this invention,generally, 100, is shown to include a cathode 102 comprising acomposition including at least one compound of formula (I) and aconventional anode 104 with an intermediate temperature electrolyte 106interposed therebetween. The electrolyte 106 can comprise a conventionalelectrolyte or a composition including at least one compound of formula(II). The cathode 102 is placed adjacent an oxygen chamber 108containing an oxygen containing gas including, without limitation, air,other nitrogen-oxygen gas mixtures, an oxygen-argon gas mixture, anoxygen-helium gas mixture, or pure oxygen so that the oxygen containinggas is in contact with one of the surfaces of the cathode 102. The anode104 is placed adjacent a hydrogen chamber 114 containing a hydrogencontaining gas including, without limitation, pure hydrogen gas, ahydrogen-helium gas mixture, or other hydrogen-inert gas mixtures sothat the hydrogen containing gas is in contact with one of the surfacesof the anode 104. The oxygen containing gas and the hydrogen containinggas can include water vapor at any desired level. Generally, the fuelcell 100 is operated at an intermediate temperature between about 400°C. and about 800° C. Generally, the fuel cell 100 is operated at or neratmosphere pressure, but lower and higher pressures can be used as welldepending on the exact application to which the fuel cell is to be used.In certain applications, higher pressure are used and in other lowerpressures are used. The oxygen chamber 108 supports a flow as indicatedby solid arrows 110 (in) and 112 (out) as does the hydrogen chamber 114,solid arrows 116 (in) and 118 (out). The fuel cell 100 also includes aresistive load 120 connected to the cathode 102 via a wire 122 and tothe anode 104 via a wire 124. The resistive load 120 can be any electricdevice capable of being powered by an hydrogen/oxygen fuel cell. Ofcourse, the effluent hydrogen gas will contain the water formed from thereaction of hydrogen and oxygen.

Results

The voltage-current (V-I) characteristics of the cell are shown in FIG.14.

As expected for CGO, the open circuit voltages (OCV) (FIG. 15) rangefrom 0.87 V at 700° C. to 1.04V at 500° C. and are lower than the Nernstpotential. The values for the OCV are comparable to those typicallymeasured with ceria-based electrolytes. The decreased OCV is attributedto the increasing contribution of electronic conductivity to the totalconductivity as the temperature is increased in a reducing environment.

The power densities as a function of current density from 500° C. to700° C. are shown in FIG. 16. The maximum power densities are rangingfrom 9.6 mW/cm² at 500° C. to 69.4 mW/cm² at 700° C. The power densitiesachievable with this cell are primarily limited by the resistance of theelectrolyte. The electrolyte is 0.8 mm thick and contributes the majorpart of the total cell resistance above 500° C. The total ohmicresistance of the cell was determined using the current interruptiontechnique. The results are compared with the calculated resistance ofCGO using literature data in FIG. 17.

At 700° C. the ohmic resistance of the cell is effectively equal to thatof the electrolyte. At 600° C., the difference is small (0.27 ohms) andincreases to 5.4 ohms at 500° C. The present experimental data do notenable the specific electrode contributions to be isolated. Neverthelessat 600° C. the data indicate low electrode resistances.

Initial tests of the performance of PBCO as a cathode material werecarried out in a complete cell. A composite electrode of PBCO/CGOsupported on a thick (0.8 mm) CGO electrolyte. The results as expectedare dominated by the electrolyte resistance but at 600° C., the ohmicresistance of the other components is only ˜0.27 ohms.

Referring now to FIG. 18, a block diagram of an oxygen separationapparatus of this invention, generally 200, is shown to include aconventional anode 202 and a conventional cathode 204 with an oxygendiffusion solid phase membrane 206 interposed therebetween. The membrane206 can comprise a composition including at least one compound offormula (I) and optionally a secondary component such as GCO (galliumdoped cerium oxide) or an inert filler to stabilize the membrane 206during starting (heating to operating temperature) and stopping (coolingto ambient temperatures). The inert filler is adapted to compensate forthe thermal expansion coefficient of the compounds of formula (I). Theanode 202 is placed adjacent a first gas chamber 208 containing anoxygen containing gas including, without limitation, othernitrogen-oxygen gas mixtures, an oxygen-argon gas mixture, anoxygen-helium gas mixture, or flue gases, from which a purified oxygengas is desired. The oxygen containing gas can include water vapor at adesired level. Generally, the oxygen containing gas is at an elevatedpressure at a temperature between about 400° C. and about 800° C.Generally, the elevated pressure is between about 1 atmosphere and 20atmospheres. In certain embodiments, the elevated pressure is betweenabout 5 atmospheres and 15 atmospheres. In other embodiments, theelevated pressure is between about 7.5 atmospheres and 12.5 atmospheres.The cathode 204 is placed adjacent a purified oxygen receiving chamber210. The chamber 210 is maintained near atmospheric or ambient pressureor slightly below atmospheric pressures to increase the pressuredifferently across the membrane. The chamber 208 can support a flow,solid arrow, of purified oxygen out of the chamber 208, while thechamber 210 can support an air flow into and out of the chamber 210,solid arrows.

All references cited herein are incorporated by reference. Although theinvention has been disclosed with reference to its preferredembodiments, from reading this description those of skill in the art mayappreciate changes and modification that may be made which do not departfrom the scope and spirit of the invention as described above andclaimed hereafter.

We claim:
 1. A cathodic composition for intermediate temperature solidoxide fuel cells comprising at least one compound having the generalformula (I):(ABO₃)_(p)(A′BO_(2+x))_(q)(A′O_(2+x))_(r)  (I) where p, q, and r areintegers, x is a real number having a value greater than 0.0 and lessthan 1.0 or 0.0<x<1.0, A is a divalent metal or mixture of divalentmetals, A′ is a trivalent metal or mixture of trivalent metals, B is atransition metal or mixture of transition metals, the three distinctbuilding blocks: ABO₃, A′BO_(2+x) and A′O_(2+x) are combined indifferent ways according to the following rules: p≧q and q≧r, where thecompounds have high oxygen activation activity and high oxygen mobilityat an intermediate temperature between about 400° C. and about 800° C.reducing an operating temperature of intermediate temperature fuel cellscurrently operating at temperatures above 800° C., where the trivalentmetal ions A′ are readily convertible between their plus three oxidationstate and their plus four oxidation state, but in a lattice of thecomposition, the trivalent metal ions A′ are rendered not readilyconvertible between their plus three and plus four oxidation states,where the transition metals B are all square pyramidally coordinated,and where ordered A cations localize oxygen vacancies into layersimproving conductivity and oxygen diffusivity at the operatingtemperature.
 2. The composition of claim 1, wherein A is selected fromthe group consisting of Ba, Sr or Pb or mixture or combinations thereof,A′ is selected from the group consisting of Y, La, Ce, Pr, Nd, Pm, Sm,Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu or mixtures or combinationsthereof, B is selected from a first transition elements excludingscandium, titanium, and zinc or mixtures or combinations thereof.
 3. Thecomposition of claim 1, wherein B is selected from the group consistingof manganese, iron, cobalt and nickel or mixtures or combinationsthereof.
 4. The composition of claim 1, wherein B is selected from thegroup consisting of iron, cobalt and mixtures or combinations thereof.5. The composition of claim 1, wherein B is iron.
 6. The composition ofclaim 1, wherein B is cobalt.
 7. The composition of claim 1, wherein pranges from 1 to
 4. 8. The composition of claim 7, wherein if p is 4,then q ranges from 1 to 4 and r ranges from 0 to 4 so that p≧q and q≧rand p≧r.
 9. The composition of claim 7, wherein p is greater than q, qgreater than r, and p is greater than r.
 10. The composition of claim 1,wherein A is Ba, A′ is selected from the group consisting of Y, La, Pr,Nd, Sm or mixtures or combinations thereof and B is selected from thegroup consisting of Fe, Co, or mixtures or combinations thereof.
 11. Thecomposition of claim 1, the compound of formula (I) are selected fromthe group PrBaCo₂O_(5.5+x), PrBaFe₂O_(5.5+x), NdBaCo₂O_(5.5+x),NdBaFe₂O_(5.5+x) and mixtures or combination thereof.
 12. Thecomposition of claim 1, the compound of formula (I) are selected fromthe group PrBaCo₂O_(5.5+x), PrBaFe₂O_(5.5+x), and mixtures orcombination thereof.
 13. The composition of claim 1, the compound offormula (I) are selected from the group NdBaCo₂O_(5.5+x), andNdBaFe₂O_(5.5+x) and mixtures or combination thereof.
 14. Thecomposition of claim 1, the compound of formula (I) is PrBaCo₂O_(5.5+x).15. The composition of claim 1, the compound of formula (I)NdBaCo₂O_(5.5+x).
 16. The composition of claim 1, wherein theintermediate temperature is between about 400° C. and about 700° C. 17.The composition of claim 1, wherein the intermediate temperature isbetween about 400° C. and about 600° C.
 18. The composition of claim 1,wherein the intermediate temperature is between about 450° C. and about800° C.
 19. The composition of claim 1, wherein the intermediatetemperature is between about 450° C. and about 700° C.
 20. Thecomposition of claim 1, wherein the intermediate temperature is betweenabout 450° C. and about 600° C.